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This paper presents a dynamic study of fluid flow around two circular cylinders placed in a transverse arrangement near a solid wall. The flow equations are solved by finite element method for Reynolds number equal to 200. The effect of geometric ratios representing the distance between the two cylinders () and the distance between the bottom cylinder and the wall () on the hydrodynamic behavior of the fluid is investigated. First, it is found that the mean drag coefficient of the bottom cylinder is larger than that of the top cylinder when () varies. However, for the top cylinder it decreases and this coefficient decreases when () increases. Secondly, it has been observed that () strongly influences the lift coefficient of the bottom cylinder. For small values of (), we observe that the values of CL2 tend to zero compared to those of CL1 with values varying between 0.2 and 0.3. Finally, it is also found that Strouhal number values of the bottom cylinder near the wall vary between 0.03 and 0.06. This is radically different from the case without wall where Strouhal number values vary between 0.16 and 0.23. [ABSTRACT FROM AUTHOR]

Abstract: A passive flow control technique, which is to drill a horizontal hole going from the front surface to the rear surface or an inclined hole going from the front surface to the top surface inside a circular cylinder of an aspect ratio H/D = 1, is proposed to control the rear recirculation region and the vortices near the free end surface. Here, both the diameter D and height H of cylinder are 70 mm. PIV measurement was performed with Reynolds number of 8570 in a water tunnel. Furthermore, in order to consider the effect of the hole position of the front surface, the cylinder models having different hole height of front surface from wall were tested. It is found that the rear recirculation zones of the horizontal hole cylinders are smaller than that of the standard cylinder. The Reynolds shear stresses and the turbulent kinetic energy are evidently reduced by the flow issued from the horizontal hole. Meanwhile, the instantaneous large-scale vortical structures of the rear recirculation zone are broken down into several small-scale vortices with decreasing the height of the horizontal hole. Although the rear separation zone of the inclined hole cylinder increases, the recirculation region near the free end surface decreases. The areas of large Reynolds shear stress and high turbulent kinetic energy increase in the rear recirculation zone. However, with increasing the height of the inclined hole, the Reynolds shear stress decreases.Graphical Abstract: [ABSTRACT FROM AUTHOR]

To reveal the possibly existed flow phenomena that buried in a two-dimensional symmetric triangle wake, one-dimensional orthogonal wavelet transform and POD are combined to decompose the fluctuating velocity field into different wavelet components and modes. The features of reconstructed flow fields are analyzed in terms of fluctuating energy, time frequency distribution, space correlation and Reynolds shear stress. It is found that the first two wavelet components and POD modes can give representations to the most energetic large-scale structures, contributing about 77 and 73 % to the total fluctuating energy, respectively. Comparing with the first two wavelet components, the first two POD modes are more appropriate to represent the Karman-like vortical structures. The time-frequency and length scale characteristics of wavelet components suggest that frequency behavior can reflect the spatial-related length scale and the wavelet transform can be used to extract turbulent structures of different scales. Similar to the energy distribution, the most significant contributions to the Reynolds shear stresses comes from the large-scale structures that composed of first two wavelet components and POD modes, accounting for 88 and 80 % of the measured maximum Reynolds shear stress, respectively. Due to different classification criterion of wavelet and POD analyses, the combining use of these two methods tends to be more effective for analyzing multi-scale turbulent structures. Graphical abstract: [ABSTRACT FROM AUTHOR]

A quasi-steady-state approach to the prediction of fluid forces acting on an insect wing leads to errors that suggest that flight is impossible. However, because biological flight does occur, the effect of unsteady fluid forces must be important to flight. The propulsive forces of these animals are related to unsteady fluid forces accompanied with the movement of vortices. Therefore, unsteady fluid forces must also be considered when estimating the propulsive force of swimming the front crawl and the flat scull technique in synchronized swimming. The purpose of this study was to investigate the effect of a three-dimensional airfoil shape on three-dimensional vortex structure for a high reduced frequency. The vortex structure and its behavior for a discoid airfoil were investigated. Furthermore, the vortex structure of the discoid airfoil was compared with that of a rectangular. The vortical flow fields were measured in the wake of the discoid airfoil using a stereoscopic particle images velocimetry technique. The three-dimensional vortex structure was depicted by plotting iso-vorticity surface calculated from the reconstructed velocity data. Compared with the rectangular airfoil, a strong vortex exists over a prolonged period of time in the wake of the discoid airfoil. Graphical Abstract: [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]